Previous to the year 1871 few observations had been made in Belgium for determining the elements of terrestrial magnetism, if we except the series which has been carried on without interruption at the Royal Observatory since 1828. Before this latter date the Intensity and Dip had never been ascertained, and there existed only two reliable measures of the Declination, viz. that of 20°35'·5 for Ostend, which Pigot observed in 1772, and the other for Nieuport, which, at about the same date, was found by Mann to be 19°48'·5. Since 1828 the observations made at any other station besides Brussels have not been numerous. In 1854 the Dip was measured at Antwerp, Courtray, Ghent, Mons, and Ostend; the Horizontal Force was found at Liége and Louvain in 1829, 1850, and 1854, and also at Namur in 1829; and the three elements were observed in 1859 at Ghent and Mechlin. The results of these various observations are collected in the work entitled “La Physique du Globe,” by M. A. Quetelet, and in Dr. Lamont’s 'Untersuchungen über die Richtung und Stärke des Erdmagnetismus in Belgien,’ &c. The above being the only determinations of the magnetic elements, there is an obvious want of a complete series of observations at a sufficient number of stations, and the survey which forms the subject of the present paper was undertaken with the view of supplying the required series of connected values of the three elements. The instruments employed in this survey were the Barrow dip-circle, the Jones unifilar, and the Frodsham chronometer of Stonyhurst Observatory, and an excellent theodolite by Troughton and Simms for determining the azimuth of the fixed points for the Declination. For this last instrument I was indebted to the kindness of James Shoolbred, Esq., C. E. All necessary information respecting the magnets and instruments will be found in the paper on the Magnetic Survey of the West of France printed in the Phil. Trans, of 1870.